The present invention relates, firstly, to a method in an internal combustion engine with at least one cylinder, a crankshaft and at least one inlet valve and one exhaust valve for each cylinder, of adapting the ignition timing relative to the crankshaft position to variations in engine operating conditions, and, secondly, to an internal combustion engine with at least one cylinder, a crankshaft, at least one inlet valve and one exhaust valve and a control unit comprising means for initiating ignition dependent on signals from a sensor sensing the angle of he crankshaft.
Variations in engine load and rotational speed are some known factors which determine the ignition timing, i.e. whether the spark should be advanced or retarded during the compression phase of the engine, and it is general practice to control the ignition timing as a function of these factors to achieve optimum ignition timing for different operating conditions. Another factor affecting ignition timing is the amount of exhaust gas in the engine fuel-air mixture. The larger the proportion of exhaust in the fuel-air mixture, the more difficult it will be to ignite the mixture. The exhaust in the fuel-air mixture can either be those recirculated from the exhaust side of the engine to the intake side (Exhaust Gas Recirculation=EGR), which is common practice for reducing exhaust gas emissions during certain operating conditions, or they can be a result of overlap between the opening times of the inlet valve and the exhaust valve, i.e. that period when both valves are open at the same time during the exhaust and inlet phase of the engine. A certain amount of overlap is unavoidable in modem engines, in order to achieve efficient gas exchange, but this overlap also results in so-called internal EGR, reducing the emissions in the exhaust.
The overlap is primarily determined by the cam angles of the valves and is in practice constant in most engines with hydraulic valve lifters with automatic valve play compensation. Engines with variable cam angles have been developed of late, making it possible to vary the lift curves of the valves depending on the operating making it possible to vary the lift curves of the valves depending on the operating conditions of the engine, to thereby increase engine efficiency. The valve overlap will thus be varied and with it the proportion of exhaust in the engine combustion chamber during the compression stroke. Such valve systems with variable valve times are known by, e.g., U.S. Pat. No. 5,209,202, which also describes ignition control as a function of cam angle.
In engines lacking hydraulic valve lifters with automatic valve play compensation and which have different materials in the valve mechanism and the cylinder head, e.g. steel and aluminium, with different thermal expansion coefficients, the valve play varies with varying temperature, resulting in undesired variations in valve overlap. The temperature differential from cold start to operating temperature can exceed 150xc2x0 C. during the winter, which will result in a substantially greater overlap when the engine is cold than when it has reached operating temperature.
One purpose of the present invention is, in general, to achieve a method of controlling ignition timing which takes into account not only variations in engine load and rotational speed but also variations in the lift curves of the valves.
This is achieved according to the invention by virtue of the fact that the ignition timing is varied in relation to variations in the period of time when the inlet and exhaust valves are open at the same time, by computing the ratio between combusted and non-combusted gas in the combustion chambers of the cylinders and utilizing this ratio in such a manner that the larger the proportion of combusted gas is, the earlier the ignition will be initiated.
An additional purpose of the present invention is to achieve an engine which is equipped to control ignition timing in the above-mentioned manner.
This is achieved according to the invention in an engine of the type described by way of introduction, by virtue of the fact that the control unit has means for computing the ratio between combusted and non-combusted gas in the combustion chambers of the cylinders and controlling the ignition timing so that the greater the proportion of combusted gas there is, the earlier the ignition will be initiated.
It is to be understood in this respect that advancing the spark or moment of ignition presupposes that all of the other controlling parameters for the ignition timing are unchanged. The combustion will take place under a longer time period, which is advantageous since exhaust gases cause the flame front to propagate more slowly.
The ratio between exhaust and fresh air can be measured with the aid of sensors directly in the combustion chamber, but even if this is technically possible, it is not practical outside the laboratory.
The ratio between combusted and non-combusted gas can, in accordance with the invention, be computed indirectly with the aid of the ratio between the temperature of the valves and the temperature of the cylinder heads. By measuring the temperature of the engine coolant and the airflow to the combustion chambers of the cylinders, said temperature ratio can be determined. The temperature of the cylinder head and the inlet valve is essentially equal to the coolant temperature. The temperature of the exhaust valve can, despite the fact that it is much higher, be calculated on the basis of the coolant temperature with a correction factor, which is proportional to the engine load, which in this case is equivalent to the air mass in the cylinder per stroke.